1. Field of the Invention
The present invention relates to a communication control method and a communication system to which the same is applied. More particularly, the invention relates to a communication control method for ensuring communication when a L2 loop is generated and a communication system to which the same is applied.
2. Description of the Related Art
In network, a L2 loop (broadcast storm) is known as one of very frequent failures generated due to improper connections of LAN cables (see, e.g., Japanese Patent Application Laid-Open Publication No. 2002-252625). When the L2 loop is generated, not only higher loads are applied to transmission paths of an entire subnet, but also a phenomenon called a black hole is generated, which forwards packets to a place where the loop exists.
A mechanism of generation of the L2 loop can be explained as follows. FIGS. 1A and 1B are diagrams describing the mechanism of generation of the L2 loop.
As shown in FIG. 1A, at the time of normal operation, a terminal 1 transmits a broadcast packet using an own MAC address “A” as an out going address. The packet is forwarded by each relay switch SW1, SW2, SW3 such that the packet reaches to all the terminals 2, 3 in a subnet.
At this point, each relay switch SW1, SW2, SW3 learns the outgoing MAC address “A” at a port receiving the packet. In other words, each switch learns that a terminal having the MAC address “A” exists before the port. Therefore, next time another terminal transmits a packet with the destination MAC address “A”, each relay switch can determine a port for forwarding from the result of the learning.
However, as shown in FIG. 1B, for example, if the L2 loop is generated due to an improper connection of a LAN cable at the relay switch SW3, a packet broadcasted in the subnet becomes a loop packet, and the packet is broadcasted in the subnet every time the broadcast packet is looped once.
This cycle is endlessly repeated until the loop packet is discarded for some reason, and the packet is endlessly broadcasted until the cause of the loop such as a cable loop is eliminated. Typically, broadcast packets such as an ARP (Address Resolution Protocol), RIP and NetBIOS are frequently transmitted to a subnet, and therefore, these packets are dominantly looped at the time of the L2 loop.
Especially, the ARP has a long packet length of about 64 bytes, and even the NetBIOS is a short packet having a packet length of about 200 bytes. When the L2 loop is generated, these packets are broadcasted in the entire subnet at a speed of transmission paths constituting the L2 loop. For example, if only ARP packets are looped in transmission paths of 100 Mbps, the packets are broadcasted in the entire subnet at a speed of a 140 thousand packets/sec which is a theoretical limitation value of the 100 Mbps Ethernet.
In this way, since reception of broadcast packets generates interrupting processing at end hosts and network devices on network, a high-load status of CPU utilization is generated especially in PCs or network devices executing software processing, and severe failures are generated, such as inability of operation.
Especially, a broadcast packet is often a packet requesting a response from a certain end host, such as an ARP request, and when a large amount of broadcast packets arrives at the end host which should respond, a higher-load status is further generated since a load of response processing is added.